The present invention teaches a vacuum-compatible die bonding technique suitable for mounting die within a vacuum housing for use in a ultra high vacuum (UHV) tube. It also teaches a method by which the disclosed die bond structure can generate precisely aligned planar structures such as photocathodes and CMOS anodes. An application where this invention is particularly valuable is when used with charge coupled devices (CCDS) or CMOS in a vacuum environment in which the semiconductor die (CCD or CMOS) is directly electron bombarded in an imaging system as, for example, as is disclosed in U.S. Pat. No. 4,687,922 (CCDs) or as disclosed in U.S. Pat. No. 6,285,018 B1 (CMOS). This invention is also applicable for attaching hybrid read out integrated circuit devices (ROIC) to thermally mismatched substrates such as ceramics within photocathode based vacuum tubes.
In the prior art, as described for example in U.S. Pat. No. 6,281,572, an intermediate pedestal is used between mismatched materials. The pedestal makes use of both an intermediate thermal coefficient of expansion (TCE) and a restricted area in order to limit the TCE mismatch induced strain in the finished device. The induced strain in the finished device is roughly proportional to the TCE mismatch times the change in temperature from the point at which the braze material solidified times the linear dimension of the braze. In the prior art, a braze material was chosen that remained in it's solid form throughout the tube vacuum process cycle. Typical braze materials and melting temperatures included AuSn (˜280° C.) and AuGe (˜361° C.) alloys. Modern photocathode based sensors including semiconductor devices are typically sealed under ultra-high vacuum (UHV) conditions. In order to achieve these conditions, tube components are typically baked at temperatures in excess of 200° C. in vacuum to eliminate residual gasses from tube components. Typically, the brazes are aligned using fixtures in order to accurately set the position of the semiconductor device within the tube. The fixtures are later removed and the tube is run through a UHV seal cycle. The high temperature of the braze alloys is required in the prior art to insure that the semiconductor does not move during the vacuum sealing step. The downside of the high temperature braze is that it locks in a high degree of strain in the finished assembly when it is cooled to room temperature. Another approach used in the prior art is to simply minimize the maximum linear dimension of the braze pad in order to minimize strain. A 0.050″ braze pad has been shown to work reasonably well with an AuSn braze. U.S. Pat. No. 6,507,147 describes a prior art package suitable for UHV applications that makes use of a small area braze. There is however a downside to this approach too. The use of a small pad limits the heat transfer area between the semiconductor device and the underlying substrate. In the case of electron bombarded CMOS image sensors or ROIC based CMOS anodes, heating of the sensor resulting from dissipation in the chip can and generally will result in degraded sensor performance. These difficulties in the prior art are avoided and/or considerably reduced through the use of physically compliant vacuum-compatible die bonding techniques enabling the efficient mounting of die within the vacuum area in a UHV tube.
In prior art UHV tubes such as the one described in U.S. Pat. No. 6,281,572, electrical connection between the semiconductor device and the inside of the vacuum envelope has been made through the use of wire-bonds that extend from the exposed surface of the semiconductor die to the underlying surface of the vacuum package. The use of wire-bonds is a well-established, reliable way to make multiple electrical connections between a semiconductor device and a thermal coefficient of expansion mismatched package. The down side of this approach is the additional package volume dedicated to the annular ring of pads that lie outside the projected outline of the overlying semiconductor die. Through the use of low melting point physically compliant braze materials, the connection pads can be moved below the semiconductor die, thereby conserving package volume and projected area.
The accuracy and repeatability of the anode braze specifies the flatness of the die within the vacuum envelope. The flatness and accuracy of placement of the die within the package in the prior art affects the cathode to anode gap and consequently sensor performance. In the case of image intensifiers employing micro-channel plates, the issue of gap control has been addressed through the use of a physical spacer between the cathode and the next tube element, the micro-channel plate (MCP). In U.S. Pat. No. 6,847,027, losue describes a method to affix a spacer to a photocathode. The spacer is then driven into the next sensor element, in this case the MCP, as the cold weld is affected between the photocathode assembly and the vacuum envelope. losue does not detail the deflection of the internal geometry of the MCP caused by the cathode contact. However, from losue's drawings it appears that the MCP will deform in response to force applied by the photocathode. MCPs are thin and, within limits, fairly flexible elements. Consequently, when supported only at the edge they can deform by a few mils thereby affecting an alignment with the photocathode, without breaking. Although this approach works reasonably well for image intensifiers, where the MCP is only supported at its edge, the forces involved would result in significant internal damage if attempted on devices that present a braze supported surface such as an electron bombarded active pixel sensors (EBAPS). Another shortcoming in losue is that the mechanism to space the MCPs and the photocathode, does not, assure that the surface of the MCP and the surface of the output window are positioned as close as possible and in a parallel relationship. Although great care and elaborate fixturing are used to achieve uniform input and output gap spacing, tolerances inevitably lead to non-uniformities. Without uniformity, the created output image quality will change as a function of position.